Atropine sulfate 0.04 mg/Kg i.v. and propranolol 0.2 mg/Kg i.v. were administered to patients with SSS in an attempt to determine intrinsic heart rate (IHR). Sinus node recovery time (SNRT) was determined before and after autonomic sympathetic and parasympathetic blockade. This method could distinguish SSS patients in two groups: group I was composed of patients with extrinsic sinus node dysfunction, group II was composed of patients with intrinsic sinus node dysfunction. We conclude that this differentiation is of great importance for clinical-prognostic evaluation and therapeutic program in SSS.